U.S. Published Application 2005/0020926A1 discloses a scanning beam imager which is reproduced in FIG. 1 herein. This imager can be used in applications in which cameras have been used in the past. In particular it can be used in medical devices such as video endoscopes, laparoscopes, etc.
FIG. 1 shows a block diagram of one example of a scanned beam imager 102. An illuminator 104 creates a first beam of light 106. A scanner 108 deflects the first beam of light across a field-of-view (FOV) to produce a second scanned beam of light 110, shown in two positions 110a and 110b. The scanned beam of light 110 sequentially illuminates spots 112 in the FOV, shown as positions 112a and 112b, corresponding to beam positions 110a and 110b, respectively. While the beam 110 illuminates the spots 112, the illuminating light beam 110 is reflected, absorbed, scattered, refracted, or otherwise affected by the object or material in the FOV to produce scattered light energy. A portion of the scattered light energy 114, shown emanating from spot positions 112a and 112b as scattered energy rays 114a and 114b, respectively, travels to one or more detectors 116 that receive the light and produce electrical signals corresponding to the amount of light energy received. Image information is provided as an array of data, where each location in the array corresponds to a position in the scan pattern. The electrical signals drive a controller 118 that builds up a digital image and transmits it for further processing, decoding, archiving, printing, display, or other treatment or use via interface 120.
Illuminator 104 may include multiple emitters such as, for instance, light emitting diodes (LEDs), lasers, thermal sources, arc sources, fluorescent sources, gas discharge sources, or other types of illuminators. In some embodiments, illuminator 104 comprises a red laser diode having a wavelength of approximately 635 to 670 nanometers (nm). In another embodiment, illuminator 104 comprises three lasers: a red diode laser, a green diode-pumped solid state (DPSS) laser, and a blue DPSS laser at approximately 635 nm, 532 nm, and 473 nm, respectively. Light source 104 may include, in the case of multiple emitters, beam combining optics to combine some or all of the emitters into a single beam. Light source 104 may also include beam-shaping optics such as one or more collimating lenses and/or apertures. Additionally, while the wavelengths described in the previous embodiments have been in the optically visible range, other wavelengths may be within the scope of the invention. Light beam 106, while illustrated as a single beam, may comprise a plurality of beams converging on a single scanner 108 or onto separate scanners 108.
One example of these scanners employs a MEMS scanner capable of deflection about two orthogonal scan axes, in which both scan axes are driven at a frequency near their natural mechanical resonant frequencies. In another example, one axis is operated near resonance while the other is operated substantially off resonance. Such a case would include, for example, the nonresonant axis being driven to achieve a triangular, or a sawtooth, velocity profile as is commonly utilized in cathode ray tube (CRT) devices and discussed in more detail later. In such cases, there are additional demands on the driving circuit, as it must apply force throughout the scan excursion to enforce the desired velocity profile, as compared to the resonant scan where a small amount of force applied for a small part of the cycle may suffice to maintain its sinusoidal velocity profile.
In a resonant scanning beam imager (SBI), the scanning reflector or reflectors oscillate such that their angular deflection in time is approximately a sinusoid, at a mechanical resonant frequency determined by the suspension stiffness and the moment of inertia of the MEMS device incorporating the reflector. Herein this mechanical resonant frequency is referred to as the “fundamental frequency.” Motion can be sustained with little energy and the devices can be made robust when they are operated at the fundamental frequency. However, sinusoidal angular deflection is less than optimal for certain applications. The varying velocity inherent in a sinusoidal scan gives varying exposure at a given point in the FOV, thus sensitivity varies with position. Achieving a desired dynamic range and resolution is most problematic in the center of the scan domain because the beam angular velocity is greatest there, requiring higher signal processing bandwidth in order to sustain a required spatial resolution at the target or scene. Therapy based on energy delivery may be least effective there and require compensating modulation. Finally, if the illumination is by laser, the power allowed when the beam reverses position at each extreme of its position is much less than that allowed when it is racing through the center.
By comparison, for some applications a “sawtooth” waveform might be employed, where the beam is translated at uniform velocity over the scene, with a much faster “retrace” at the end of each scan. Alternatively, a “triangle” waveform beam displacement might be employed, where the retrace occurs at the same rate as the scan in the opposite direction. FIG. 4A illustrates how beam position and angular velocity vary a sawtooth approach, and FIG. 4B illustrates the position and velocity vary in a triangular approach. In either approach, the beam velocity is uniform as it moves across the field of view, reducing the bandwidth required in the controller 118, providing more uniform performance over the field of view, and allowing a higher illuminating power level.